Goku would become aware of a plot by an old enemy of his, Emperor Pilaf, who had gathered the Black Star Dragon Balls and attempted to summon Ultimate Shenron, who was capable of even more powerful wishes. Realizing what was at stake, Goku reaches them in time to stop Pilaf's wish of conquering the world, or so he thought. In the very last moment, when even Pilaf was accepting defeat, Pilaf whispered his frustration with how powerful Goku was, accidentally wishing for the Saiyan to turn back into a kid. The wish was heard by Shenron, and thus it was granted.
As part of their immense power, the Black Star Dragon Balls also come with one major issue. If all seven balls are not retrieved and sent back to earth within a year, the planet will be destroyed. Thus, Pan, Trunks, and a childlike Goku tail and all set out to explore the universe to find the seven Dragon Balls and return them to the planet to avoid its destruction. A task made all the harder since, the younger body of Goku is unable to achieve the higher levels of power of his adult form.
Goku GT has many qualities that make him a great balance character for competitive players. For one, he has the smallest hurtbox in the game, making him hard to hit and easily see what he is doing on-screen at any given moment. Another plus is his meter management ability, plenty of moves to offload meter for great damage. His great mix-up, staggers, and high damage potential combine to put forth a very hard opponent. But there are always downsides, most of all his very short reaching normals that require very close distances, and secondary, for all his meter, he still needs it for neutral, as meterless options are limited.